Bracing for an Interplanetary Traffic Jam

The Deep Space Network

NASA's traffic control system for interplanetary
spacecraft is bracing for a flurry of activity in deep space.

June
6, 2001 -- On April 28, 2001, a weak radio signal reached
Earth from beyond the orbit of Pluto. It was NASA's Pioneer
10 spacecraft, struggling to communicate with ground controllers,
its message riding on a radio signal that registered just a billionth
of a trillionth of a watt.

How do you listen to a transmission that couldn't make a lightbulb
glow in a billion years? It's all in a day's work for NASA's
extraordinary Deep Space Network (DSN).

Right: A 70-meter antenna at the Deep Space Network
Goldstone complex in California.

The DSN is a global system for communicating with interplanetary
spacecraft. The largest and most sensitive scientific telecommunications
system in the world, it also performs radio and radar astronomy
observations for the exploration of the solar system and the
universe.

The DSN consists of three clusters of antennas spaced approximately
120 degrees apart around the world: at Goldstone, in California's
Mojave Desert; near Madrid, Spain; and near Canberra, Australia.
"The strategy here is, no matter where the spacecraft is,
you're always in contact with it," explained Statman. Each
complex is situated in semi-mountainous, bowl-shaped terrain
to shield against radio frequency interference.

Above: DSN locations in Spain, Australia, and California
are approximately 120 degrees apart in longitude, which enables
continuous observation and suitable overlap for transferring
the spacecraft radio link from one complex to the next.

The centerpiece of every DSN facility is an enormous 70-meter
diameter antenna (230-foot) capable of tracking spacecraft more
than 16 billion kilometers (10 billion miles) from Earth. Arrayed
around that dish is an assortment of 34-meter, 26-meter, and
11-meter antennas. The 26-meter antennas feature a double-axis
astronomical mount that allows them to point low on the horizon
to pick up fast-moving, Earth-orbiting satellites as soon as
they come into view. These can track at up to three degrees per
second.

DSN antennas communicate with far-flung spacecraft at radio
frequencies of 2.2 GHz, 8.4 GHz, and 32 GHz. For comparison,
the lowest frequency, 2.2 GHz, is about the same as radio waves
that cook food inside household microwave ovens.

All of the antennas communicate directly with the Deep Space
Operations Center at JPL in Pasadena, CA. The center staff directs
operations, transmits commands and oversees the quality of spacecraft
telemetry and navigation data delivered to network users.

NASA recently announced it's upgrading
the DSN to handle a surge in interplanetary traffic.

Left: NASA's Stardust spacecraft will be one of many
distant missions competing for Deep Space Network time in 2003.

"We're getting ready for a crunch period beginning in
November 2003," said Rich Miller, head of planning and commitments
at JPL. That's when the U.S., Europe and Japan all will have
missions arriving at Mars. These include NASA's
2003 Mars
Exploration Rovers, the ESA Mars
Express Mission, and the Japanese Nozomi
spacecraft. At the same time Stardust and Deep
Space 1 will be encountering comets and a third comet
mission named "CONTOUR"
will launch. And, of course, other ongoing missions will have
continuing communications needs.

"[These new] missions all happen to lie in the same part
of the sky," said Statman, who described the area where
the spacecraft will cluster as a slice of the sky with Mars in
the middle. "We need to track them but we don't have enough
antennas."

Madrid will receive a new 34-meter antenna that will increase
available spacecraft-tracking time by about 105 hours per week
when Mars is in view. The Madrid complex's current capacity is
315 hours.

"The tracking capacity is proportional to the number
of antennas at each location," said Statman. "At the
moment, Madrid is the most crucial site for an upgrade simply
because we need more tracking time there."

Goldstone already supports as many as 420 hours per week of
deep space communication, a figure that will balloon to 525 hours
when an existing antenna comes online in 2003. "Both the
Japanese and the Europeans have tracking antennas in Australia,"
says Statman, so they can help with the communications load at
that longitude.

As part of the upgrade, older hardware and software systems
will be phased out and replaced with ones that are more reliable
and, in some cases, automated. Also, Madrid and Canberra will
receive processing equipment that will allow operators to combine
signals from multiple on-site antennas, increasing their sensitivity
to distant transmissions. Goldstone can already do that.

Every bit of extra sensitivity is welcome, says Statman. The
total signal power arriving at a network antenna from a spacecraft
transmitting from the outer solar system is 20 million times
weaker than the power level from a modern digital watch battery!

Teasing
out faint signals from space probes isn't all the DSN does --
it's a powerful scientific instrument in its own right. The Goldstone
70-meter antenna, for example, doubles as a powerful solar system
radar. It captures radar images of planets and passing asteroids,
searches for water on the Moon, and helps pick landing sites
on Mars. Together, the three DSN facilities along with other
antennas around the world form a powerful Very Long Baseline
Interferometer that can peer into the hearts of quasars, measure
Earth's continental drift -- even test general relativity.

Above: Astronomers used the Goldstone radar to image
near-Earth asteroid 1999 KW4 when it passed by Earth last month.
They discovered the space rock was a binary! [more
information]

Not bad for a bit of moonlighting!

NASA's Deep Space Network truly is an international treasure,
and it's about to become even better. For more information about
DSN and its ongoing upgrades, please visit the
Deep Space Network home page from JPL.

Would you like to use this
story in your 6th to 12th grade classroom? These lessons might
help:

Ears to the Sky: Stop! Before your students read this article,
let them take an amusing pre-reading quiz to clarify the meaning
of terms like 'Dish' and 'Deep Space.' Then dive into some serious
discussion questions later. [lesson
plan] [pre-reading]
[post-reading]

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